Biomedical subjects

P Neogi

Publications and source records attributed to P Neogi.

12 recordsLinked to original sources

PubMed2006

On one-dimensional self-assembly of surfactant-coated nanoparticles.

Nanometer-sized metal and semiconductor particles possess novel properties. To fully realize their potential, these nanoparticles need to be fabricated into ordered arrays or predesigned structures. A promising nanoparticle fabrication method is coupled surface passivation and self-assembly of surfactant-coated nanoparticles. Due to the empirical procedure and partially satisfactory results, this method still represents a major challenge to date and its refinement can benefit from fundamental understanding. Existing evidences suggest that the self-assembly of surfactant-coated nanoparticles is induced by surfactant-modified interparticle interactions and follows an intrinsic road map such that short one-dimensional (1D) chain arrays of nanoparticles occur first as a stable intermediate before further assembly takes place to form higher dimensional close-packed superlattices. Here we report a study employing fundamental analyses and Brownian dynamics simulations to elucidate the underlying pair interaction potential that drives the nanoparticle self-assembly via 1D arrays. We find that a pair potential which has a longer-ranged repulsion and reflects the effects of surfactant chain interdigitation on the dynamics is effective in producing and stabilizing nanoparticle chain arrays. The resultant potential energy surface is isotropic for dispersed nanoparticles but becomes anisotropic to favor the growth of linear chain arrays when self-assembly starts.

Journal Article↗

PubMed2006

Solubilization kinetics of soluble oils by an ionic surfactant.

Previously available data on solubilization kinetics of two oils in a micellar solution of sodium dodecyl sulfate, have been plotted with the data for one more oil provided here. These rates show that they reach saturation values with increasing micellar concentrations. These saturation values are then shown to be linearly dependent on the solubilities of oil, in keeping with the theory that the mechanism for solubilization of soluble oils by an ionic surfactant is chiefly molecular dissolution of the oil followed by a quick uptake by the micelles.

Journal Article↗

PubMed2006

Wagging the contact line: transverse and longitudinal waves.

Kinetics of wetting has been explored where the contact line not only sees a steady spreading but also has longitudinal or transverse oscillations imposed on it. The latter case is realized when spreading takes place over a rough surface. The effects of the imposed motion are small, which seem to be due to low spreading rates and small dynamic contact angles used in this study. However, a singularity is seen in viscous dissipation during the movement on the model rough surface, which is interpreted here as an instability that is similar to Haines' jumps and stick-slip phenomena, with possible entrainment of the displaced fluid. This is the first time that all of these have been associated with each other.

Journal Article↗

PubMed2005

Wetting kinetics of films containing nonadsorbing polymers.

Kinetics of wetting by a polymer solution have been studied theoretically for a film pinned to a slot. The fluid mechanical equations have been solved using a numerical scheme. The role of polymers appears in the disjoining pressure in the model. The spreading kinetics are observed to follow a power law: a power of 14 is observed at short times due to the Laplace pressure, and 12 at large times under the Hamaker part of the disjoining pressure at very large times and with no equilibration. It is argued and demonstrated that techniques which have low resolutions such as microscopy will measure quite different kinetics: at short times a power of 14 as for wetting liquids and then a sudden equilibration as reported in these experiments. It is also argued on the basis of steric exclusion, and quantified in the disjoining pressure, that the behavior returns to that of wetting liquids when the polymer molecular weight becomes very high, as also observed in the experiments. Examples of how these features can find practical applications, and hence, the importance of use of polymers as additives are given.

Adsorption↗

PubMed2004

Interfacial stability of electrodeposition of cuprous oxide films.

Experiments on deposition of Cu(2)O films from basic copper sulfate solution show that copper also deposits. At low, but basic values of pH only copper deposits and at high pH only cuprous oxide deposits. In the intermediate range where both compete the system shows oscillations at "constant current." Linear stability analysis has been conducted for such an electrochemical cell to show that oscillations can take place in the parameter space identified in the experiments. The results are keeping with most of the experimental observations, which are many, but not with all. The physical mechanisms behind the oscillations are explained in terms of competing reactions.

Journal Article↗

PubMed2004

Dynamic contact angles under evaporation.

Wetting kinetics in the presence of heat transfer, evaporation, and Marangoni effect has been explored using a method used by Joanny and de Gennes [C. R. Acad. Sci. Ser. II 299, 279 (1984)]. The method solves for the dynamic contact angle alpha by equating the rate of surface work to the rate of dissipation. The result in the form of dynamic contact angle alpha as a function of capillary number Ca agrees well with existing experimental data. Qualitative comparisons have been made in cases where quantitative data do not exist. Finally, a form of contact line instability is predicted, for which some experimental justifications are seen.

Journal Article↗

PubMed2003

Stable drop shapes under disjoining pressure. III. Sharp contact lines.

The shape and stability of a drop with a sharp contact line are analyzed. The disjoining pressure is included. The choice made for the latter is special in that it admits a sharp contact angle as appropriate in this case. The stability analysis is considered for two scenarios, one where the contact line cannot be moved which mode turns out to be stable to small perturbations, and the other where it can be moved but the contact angle is fixed at the equilibrium value. This second case turns out to be unstable to small perturbations due to the curvature effect of the drop.

Journal Article↗

PubMed2003

A model for interfacial resistance observed during solubilization with micellar solution.

The quantitative behavior of interfacial resistance to mass transfer at an oil-water interface in presence of a surfactant, which occurs in addition to the diffusional resistance, is modeled. Successive approximations are used to simplify the detailed behavior to show that the experimentally determined quantitative behavior of solubilization kinetics, is also theoretically justified. The solubility of oil, the frequency of collision of micelles with the oil-water interface, and the resistance to the entry of the solubilizate into the micelle (activation energy) are identified as the key quantities governing this resistance.

Journal Article↗

PubMed2002

Interfacial resistance in solubilization kinetics.

An experiment has been designed to measure the kinetics of solubilization of oils in a micellar solution. The results show a very large range of behaviors, but in one system the solubilization rate is determined completely by what is called the interfacial resistance, which quite likely arises out of the structural changes that micelles see. The model given here is successful in showing that this resistance is not the usual resistance due to diffusion and that it is quite prelevant in systems that are chemically similar but other conditions varying widely (Appendix). We conclude by showing that some of the existing theories of interfacial resistance are not reasonable, suggest how the real system may be more complex, and show that the interfacial conductance k* proportional c(m)(-1) at large surfactant concentrations where c(m) is the concentration of micelles.

Journal Article↗

PubMed2002

Stable drop shapes under disjoining pressure. II. Multiplicity and stability.

The stability of the contact line region as affected by the disjoining pressure has been analyzed by solving the augmented Young-Laplace equation. Because of the results in Part I (Zhang, X., Neogi, P., and Ybarra, R. M., J. Colloid Interface Sci.), we have concentrated on obtaining multiple solutions for the same set of conditions. As many as five solutions were obtained: drops that end in a thin film with uniform thickness and where the film shape oscillates, drops that end with microscopic contact angles, as well as uniform thin films of two different thicknesses. The results of linear stability analysis were used to show that most cases were unstable to infinitesimal disturbances. Only two stable drop shapes for the particular disjoining pressure investigated are stable, a thin film of constant thickness and a thin drop that ends in a film of same thickness. Both multiplicity and stability have been discussed here for the first time and shed considerable light on the role of the attractive and repulsive forces.

Journal Article↗

PubMed2002

Stable drop shapes under disjoining pressure. I. A hierarchical approach and application.

The contact line region as affected by the disjoining pressure has been analyzed under the assumption that it can sustain only two types of profiles. Disjoining pressure represents the extra potential in thin films that always exists in the contact angle region where a liquid drop or a wedge thins to meet the solid substrate and in turn affects the contact angle as well as the film profile. It is shown here that the integration of the augmented Young-Laplace equation to yield the above types of drop profiles under the action of disjoining pressure leads to the usual conditions of equilibrium as well as the condition of stability in the same analysis. Other inequality constraints are obtained where the stability condition does not apply. The fact that stability condition coexists with the conditions of equilibrium is pursued to show in one case that the stability modifies half of the predicted outcomes in the drop shapes. In addition, exceptions to the rule are found, which are physically meaningful, and a scale-dependent equilibrium is reported for the first time.

Journal Article↗

PubMed2000

Total synthesis of asimicin and bullatacin.

The efficient total synthesis of asimicin, 1, and bullatacin, 2, has demonstrated the advantages of three different strategies for the synthesis of the tricyclic intermediates 6 and 7, which represent the key fragment of the bis-THF Annonaceous acetogenins. The naked carbon skeleton strategy is based on the production of all asymmetric centers by selective placement of the oxygen functions onto an unsaturated, nonfunctionalized carbon skeleton. Diversity in this approach arises from the relative timing of highly stereoselective reactions, such as the Sharpless asymmetric dihydroxylation (AD) reaction, the Kennedy oxidative cyclization (OC) with rhenium(VII) oxide, the Mitsunobu-type alcohol epimerization reaction, and the Williamson etherification reaction. The convergent strategy, which is based on the combinatorial coupling of two series of diastereomeric fragments, to produce intermediates such as 11 and 12, enjoys the advantages of both efficiency and versatility. The third approach, which is based on partially functionalized intermediates, such as 13, combines the advantages of both the linear and the convergent strategies-synthetic efficiency and diversity.

Antineoplastic Agents, Phytogenic↗